Photocell for measuring long wave radiations



Aug. 20, A1946. C, G, FlNK TAL 2,406,139

PHOTQGELL' FOR MEASURING LONG WAVE RADIATIONS l Filed Feb.. 27, 1941SWW' E ATTORNEY La rrOWS- j Figure? is" agdiagrammatic View, showinghow. radiations from an infrared source may be '5 Patented Aug. 20, 1946T oFElcE PHoToCELL Fon MEASURING LONG WAVE RADIATIoNs Colin l(Br. Fink,New York, N. Y., andJohnstone S.,

Mackay, Prospect Park, Pa.

Application YFebruary 27, 1941, Serial No..380,868

' 7 Claims. Y 1 This invention relates to photoelectric cells,

and more particularly to such for measuring long Wave or infraredradiations.

The principal object of our invention, generally considered, is theproduction of a photocell comprising a solid photo-element, bismuthsulfide, bismuth selenide, or equivalent, which is adapted toefficiently measure infrared rays including those of relatively longwave lengths.

Another object of our invention is the production of a photocellemploying an element which has a large proportion or nearly all of thisactivity due to infrared radiations.

A further object of our invention is Ito develop a photocell whichefficiently responds to infrared 1 radiation, as by developing as muchas five microamperes per lumen, or the equivalent in microwatts, ofenergy received.

A still further object of our invention i-s the employment of bismuthsulfide for measuring ini frared radiations to 70,000 Angstrom units,With only a small response to visible radiations and those of shorterWave length.

Other objects and advantages of the invention, relating to theparticular arrangement and construction of the various parts, willbecome apparent as the description proceeds.

Referring tothe drawing illustrating our invention: Y

Figure 1 is a transverse sectional view of a simple form .ofphotoeleotric cellpembodying our invention.

Figure 2 is a face View the left in Figure 1.

Figure 3 is a view of a cell, such as shown in Figure 2, `embodying agrid for increasing the conductivity of the photosensitive layer.

Figure 4 is a View of a cell, such as shown in Figure 1, .thephotosensitive portion 'of which is, however, duplicated on" ltheback sothat it is adapted to receive radiations from both sides.

Figure 5 is a face View of a cell, such as shown in Figure 2, exceptthat it is of the null reading or compensating type.

.FigureV 6 is a view showing how a cell, such as illustrated in Figure1, may be adjustably mounted in position and associated with a'lter.

Figure 7 is an elevational view of the com-` bination of a cell,embodying our `invention,with a directional shield.

-Figure 8 is a vertical sectional view on `they line VlIlI--VVIII ofFigure 7 in the direction of they of `the cell looking from 2transmitted from a distance and concentrated on acell, embodying ourinvention, for increasing the sensitivity thereof..

Referring to, the drawing in detail, and first considering theembodiment of our invention illustrated in Figures l and 2, our cell I Iconsist-s essentially of a soft metal backing I2, de` sirably of tin,covered with a layer I3 of a bismuth compound, such as the sulfide,(BizSs), and the selenide (BizSes), which is in turncoated with a thintranslucent film I4 of a conductor such as copper, silver, cadmium,carbon, bisv muth, lead, tin, 'combination of tWo or more `of the metalsmentioned, or equivalent material.

Bismuth sulfide A(or equivalent) of such a `cell is a solidphoto-element corresponding to the cuprous oxide or selenium of therectifier type of photocell. Such a cell generates its oWn power underthe inuence of radiations, Without `the external application ofelectromo-tive force. Both the direction of response and the efficiencyof such a photocell, are dependent on the characteristics of thephotosensitive material which might be described as asemi-conductor. Y

The distinguishing feature of our cell, is that it is sensitiveltoenergy in the infrared outto a threshold wave length of 70,000 Angstromunits. By comparison, the selenium cell is sensitive to energy out to8,000 Angstrom units, caesium to 15,000 Angstrom units, and cuprousoxide to 14,000 Angstrom units. 4 f

Although it is possible to detect infrared radiations having wavelengths of 15,000 Angstrom units by means of a thermopile, which is avery expensive and delicate instrument, our invention makes directreading of Ithe long Wave length infrared radiations cheap andpractical. l'

The bismuth sulfide layer I3 may be about".005

inch in thickness, .although thi-cker films have' been used. It isapplied to the tin backing I2 under high pressure, such as about ftytons per square inch. Other metals besides tin-may be employed ifdesired, but tin has been'found satisfactory. We prefer to make thebismuth sulfide by chemical precipitation With hydrogen sulfide fromacid solution, avoiding an excess of bismuth or sulfur. Such procedureis Well known to all chemists. An acid treatment after preparation hasbeen found desirable. We have, however, tried other methods ofpreparationybu-t the one mentioned has given goodresults.

When the bismuth sulfide is firmly pressedinto and onto the tin or.other soft'metal backing plate .I 2,"5- the f outer surface Vof l:the fphot'olayer Vis covered with a thin layer of infrared transmittingV orcompensated cell may be constructed, as `shox post 29. A set screw 30,holds the cell tight and position from an acid sulfate rof copper bathor by sputtering the material selected from the list previously given.As an alternative, it has been i found satisfactory to rub a conductingvery thin layer of graphite on the surfaceaof the photol layerV I3. Sucha 'coating is'too thinfto measure 1 with a micrometer, and has beenfound to transmit about 20% of the radiation from an incandescenttungsten lament lamp, as mea'sureclby a selenium cell.

In order to connect a cell produced, as shown 1 i in Figure 2, with ameasuring instrument such asa milliammeter i5, a ring I of metalor-other l 1 conducting material is .desirably applied, as

g, shown, in engagement with the translucent layer I4, in order to makegood contact there-K with, and the instrument I5 connectedbetween isaidring I6 and the soft'metal base, as by leads I'I and I8.' v

The electrical resistances of cells, produced in j accordance ywith ourinvention, have been Vfound to vary considerably.

That is, they may be as lc'iv as'75 ohms and ashigh as 15,600 Ohms,depend` ing on the thickness of bismuth 'sulde the method ofpreparaticn,and thekind of contacts made to it.

.In order to marke g-ocd contact, a metallic layer l may besputtered orelectrically deposited, as in,- dicated at I in Figure rl, land'inorderto cut down the cell resistance, lmultiple contacts may be made with thetranslucent layer Ill by a gridlike structure I connected to a4peripheral ring Isa, like the ringv I6 of 'Figures land A2, as shown 1in Figure 3.-

provides an electrical connection with the metal base |29. Leads IIe andI8e extend, respectively, from the binding post 29 and screw 30, to themillammeter or other measuring instrument, not

shown.

The filter 25, shown covering .the cell face in order to limit the range:of infrared radiations to be detected, may be formed of Pyrexl glass,which will pass infrared up to about 18,000

.Angstrom units, heat transmitting glass which transmits rays between8,000 and 40,000 Angstrom units, or hard rubber which is a goodtransmitter of heat rays.

It is also desirable, inrmany cases, to enclose the cell in .order toprotect it fromY fumes, dust or other deteriorating action, in whichcase any infrared transmitting substance may be used over the frontface, such as one of the lters given above, and hermetically 'sealed tothe box.

Asfit is desired to avoid heat from extraneous sources, which mightvitiaterthe results, 'the box Figure l illustrates a further'ernbodimentof` our invention in whichvthemetal backing plate I2"b is inthecenter,:.and'both surfaces-are coat-f` y ed with layers 93h of.bismutnrsulde Vthe outer 1 Vsurfaces of -which layers vare then, in.turnfeacl-.r 4coated with atranslucent"cenductinfnlm isb; The filmsarethendesirablylconncted in parai= lel, as by means of .conductorhthrough con- Y necting rings lh. and themilliamnieter or otherindicating Yinstrument may be connected to 21% enclosing the cell isdesirably formed-of heatinsul-ating material, such a's Celotex. Y

In infrared signalling, a highly udirectional effect is often desired.vlin such a case, afshield .3i of insulating and heat absorbing materialmay be used with a bc-X 24F holding-a bismuth suliide `cell Hf, as shownin Figures 7 .and-8. ln this case the iront contact 22. is provided by;the box itself, and the backcontact bythe setscrewZi-lf which engagesthe metal backing plate f lilf. box v2if may be provided with a-pocket25f for the reception rof ia iilter, like the lilter 25 of Figure .6, ifdesired.-

IAsan example of howdirectional effects may in the form of aparabolicreiiector. Y

duplex cell. by leads I'ib and Ib from one ,of the rngs"`I6b and themetal plate lh;asillustrated.`

*Forf convenience in measuring, anull rea in Figure 5, that is, twoequivalent photoelectr mountedzon a suitable base'f, `and 'balancedagainst one another through a resistance izan-:l i milliammeter or othersensitive measuring Tin After theyhave been both strumentv 23. Y posedto the same vamount and kind ofradiation,

and balanced against one another, thenthe con-1 nections may be changed'toA make them act toknown v electromot'ive force, asis usual withsu'ch` vFigure 6 illustrates a'forin' of cell I iijconstruct-` "ed asshownin'Figures 'l and?, and "adiustably` `mountedin al box 214; whereit 'may be employed Y A ybismuth.sulfide cell vi ig, constructed in-accordancewith ourinvention, is mounted-=at-the focus of a receivingdevice .34, which is alsoshown inthe form of a parabolic reilector, the`sensitive surface facing inward' or toward the reflecting surface ofthedevice 35i. 'in this way theuenergy is transmitted Afrom 'the sendingdeviceito the receiving device without much loss, and-focused f metric.

on the cell Which is then ina position to efliciently record theradiations. Y Y

Tests of bismuthsulde cells, constructed in accordance with ourinvention, rshow that y-they have good stability, small fatigue, slowresponse, and a lconductivity, which is only yslightly asym- The currentproducedis roughly-proportional to the area exposed, vinverselyproportional tothe squareof the-distance iromrthe .light source, andboth current and voltage; are

kdirectly proportional to the wattage of the light source.Y 'A

The degree of sensitivity.

Y agreat deal from cell tc cell, largely due to varia- [tions bound ,tooccur .in construction-andmore metal ba'seIZe; ai photosenis'it'ive"layer of bismuth f .v lllfide fsezfah'd translucent lol'ltll'irlffl'red-Wf'transmitting electricalV conducting'jlayer" isc; 1 Contact is'rradejwithfjthe Vlatter, #through a .graphite 4.front contact 21,mountedY otra;resilient,y

Aor ."lessV accidentalV characteristics. :The ,silver-".electroplated,bismuthfsulde*cell'which ,had a` `Io poor response,showed a relatively highiunipolar conductivity. The ratioofthe...conductivity'in Aopposite directions was te'n toi-one. -InAthea-cases of the' higher sensitivity cellsfit lwas'around two to one,although this was not instantanefusgthat is; the 'resistance'staiftedoiit about this same l as ,and` stabiiitylyahes for othercells, but would then drift down if the current Was in one direction andup if it were in the other.

The bismuth sulfide cell in Which the outer conducting or translucentlayer was formed by evaporating Woods metal thereon, showed goodasymmetric conductance and the only nearinstantaneous current responseobserved for any of such cells.

The experimental bismuth sulde cells We made Were not so photosensitiveas the seleniumcell for radiations from an incandescent tungsten lamp orfrom the sun. Also, they were not so good as a sensitive thermopile forinfrared radiations. They are much cheaper, however, than a thermopile.A cell produced in accordance with our invention can, for example, beused to indicate the amounts of energy radiated from black. bodies atlow temperatures. Thus radiations from a flat iron heated to about 450-C. gave fty microamperes when the iron was disposed cm. from the cell.

Photovoltaic cells produced in accordance with our invention should findnumerous practical applications in controlling furnaces,l detectingsignals in fog, burglar alarms, automatic re signals, sprinkler systems,chemicalreaction control device, and in any place Where it is desired tocontrol relatively low temperatures. Although the cells which We, haveexperimented with have had areas of about 1.2 square inches, it ispossible to produce cells of almost any desired surface area. Thethickness of the backing metal is not critical, but should be greatenough to stand having the sensitive coating pressed thereinto andproperly rigidify the cell. The great advantage of our cells, over othermeans for detecting infrared radiations, is that they may be con..structed at small expense, While the total photoelectric sensitivitycompares favorably with that of the vacuum tube photoelectric cells,being much greater than that of the sodium, and about the same as thatof the caesium cell.

Although preferred embodiments of our invention have been disclosed, itwill be understood that modifications may be made'within the spirit andscope of the appended claims which are not all limited to bismuthsulfide, as another compound of bismuth, such as the selenide, may besubstituted.

We claim:

1. A photovoltaic cell having about of its photoactivity in the infraredregion and comprising a conducting base, a layer of material selectedfrom the group consisting of bismuth sulde and bismuth selenidethereover, and a thin translucent electrically conducting iilm over saidlayer.

2. A photovoltaic cell sensitive t0 infrared radiations shorter than70,000 Angstrom units and comprising a conductive base, a layer ofmaterial selected from the group consisting of bismuth sulfide andbismuth selenide thereover, and a thin translucent electricallyconductive lm over said layer.

3. A photovoltaic cell having about 80% of its photoactivity in theinfrared region and comprising a tin base, a layer of bismuth suldethereover, and a thin translucent l layer of metal selected from thegroup consisting of copper, silver, cadmium, bismuth, lead, tin andcombinations of two or more of 'such metals deposited thereon.

4. A photovoltaic cell having 80% of its photoactivity in the infraredregion of the spectrum and comprising a conducting base, a layer of bis-.muth sulde, and a thin translucent electrically conducting film oversaid layer.

5. A photovoltaic cell having about 80% of its photoactivity in theinfrared region and comprising a conducting base, a layer of bismuthsulfide thereover, a' thin translucent electrically conducting lmdeposited on said layer, and a contact ring engaging said layer.

6. A photovoltaic cell having about 80% of its photoactivity in theinfrared region and comprising a conducting base, a layer of bismuthsulfide thereover, an electrically conductive translucent lm on saidlayer, and an electrical conductive grid engaging said film forincreasing the conductance of said layer.

7. A photovoltaic cell comprising a conducting base, a layer of bismuthsuliide pressed into each face of said base, and a translucent iilm ofelectrically conducting material disposed over each layer.

COLIN G. FINK. J OHNSTONE S. MACKAY.

